Bionanoconjugates were created with cytochrome c from horse heart (HCc) or yeast (YCc) and citratestabilized
gold nanoparticles (AuNPs). Evidence for the formation of stable HCc-AuNP and YCc-AuNP
bionanoconjugates came from a 5 nm red-shift of the surface plasmon resonance band of the AuNPs;
increase of the zeta-potential; and direct visualization by Atomic Force Microscopy. Langmuir isotherm
fittings of zeta-potential data indicated that higher enthalpy changes are involved in the formation of the
HCc-AuNP than in YCc-AuNP. UV-visible and circular dichroism studies of pH-induced aggregation of
the bionanoconjugates revealed distinct protonation patterns with an aggregation pH of 8.8 and 6.2, for
YCc-AuNP and HCc-AuNP, respectively. No appreciable changes were observed in the secondary
structure of HCc in HCc-AuNP. In contrast, YCc in YCc-AuNP presented a decrease in alpha-helix content
upon AuNP binding, and an increase in beta sheet content upon pH-induced aggregation. Data discussion
is based on the distinct binding modes of both proteins to the AuNPs: via a covalent bond (Cys 102) for
YCc; and via electrostatic interaction for HCc.